Punishment corresponds to guilt. Now the woman was more grievously punished than the man, as appears from Genesis 3. Therefore she sinned more grievously than the man. As stated (Article 3), the gravity of a sin depends on the species rather than on a circuмstance of that sin. Accordingly we must assert that, if we consider the condition attaching to these persons, the man's sin is the more grievous, because he was more perfect than the woman.
As regards the genus itself of the sin, the sin of each is considered to be equal, for each sinned by pride. Hence Augustine says (Gen. ad lit. xi, 35): "Eve in excusing herself betrays disparity of sex, though parity of pride."
But as regards the species of pride, the woman sinned more grievously, for three reasons. First, because she was more puffed up than the man. For the woman believed in the serpent's persuasive words, namely that God had forbidden them to eat of the tree, lest they should become like to Him; so that in wishing to attain to God's likeness by eating of the forbidden fruit, her pride rose to the height of desiring to obtain something against God's will. On the other hand, the man did not believe this to be true; wherefore he did not wish to attain to God's likeness against God's will: but his pride consisted in wishing to attain thereto by his own power. Secondly, the woman not only herself sinned, but suggested sin to the man; wherefore she sinned against both God and her neighbor. Thirdly, the man's sin was diminished by the fact that, as Augustine says (Gen. ad lit. xi, 42), "he consented to the sin out of a certain friendly good-will, on account of which a man sometimes will offend God rather than make an enemy of his friend. That he ought not to have done so is shown by the just issue of the Divine sentence."
It is therefore evident that the woman's sin was more grievous than the man's.
The woman was deceived because she was first of all puffed up with pride. Wherefore her ignorance did not excuse, but aggravated her sin, in so far as it was the cause of her being puffed up with still greater pride. In the act of tempting the devil was by way of principal agent; whereas the woman was employed as an instrument of temptation in bringing about the downfall of the man, both because the woman was weaker than the man, and consequently more liable to be deceived, and because, on account of her union with man, the devil was able to deceive the man especially through her. (Summa Theologiae Second Part of the Second Part Question 163 and ibid. Question 165)